Ensure temporary directory exists

This PR amends the #994 PR to:

reset the changes to the top-level .gitignore file, which are not needed;
have the debug file written to src/ontology/debug.log, rather than src/ontology/tmp/debug.log, thereby avoiding the bug that motivated #994 in the first place altogether.

This way, we simultaneously ensure that the src/ontology/tmp directory always exist and remove the need for that directory to exist for the debug mode to work (“belt-and-suspenders” approach).

The use of a .gitkeep file to force the existence of src/ontology/tmp is not extended to other directories such as src/ontology/mirror, src/ontology/components, src/ontology/subsets, etc. The rationale is that there is no reason for those directories to exist if they are not used (for example, an ontology that does not import any other ontology has no need for src/ontology/mirror; an ontology that defines no subsets has no need for src/ontology/subsets, etc.); by contrast, src/ontology/tmp will always be needed at some point or another.

INCATools / ontology-development-kit

Ensure temporary directory exists #1010